How Do Stratus Clouds Form?

If stratus clouds are layering the sky, it’s likely to be a gray and overcast time of day. Generally forming below 6,000 feet (about 1,830 m), stratus clouds are thin but spread over vast territory, often from one end of the horizon to the other.

To form stratus clouds, air current rises just high enough into the atmosphere to create low-hanging sheets of condensation. Whenever the temperature dips under the dew point, this vapor darkens and forms into distinctive stratus clouds that often produce rain when high humidity is present. [1]

Stratus clouds can come alone, but often form in four basic combinations, each distinct from the other in observable ways. These other related cloud forms are:

Altostratus

Forming between 6,500 and 20,000 feet (6,096 m), these flat gray clouds often cover the whole sky in repetitively forming lips, keeping any shadows from forming on the ground. Altostratus clouds, which let just a hint of the sun shine through, frequently preceded the coming of nimbostratus clouds and heavier downpours. [2]

Nimbostratus

These are rain clouds to be sure, but only light rain. Dark gray on the horizon, nimbostratus clouds regularly come after the slightly higher and lighter altostratus clouds, but are typically lower in the sky– under 6,000 feet like their white cousins, the stratus. [3]

Cirrostratus

Cirrostratus clouds are thin and higher in the sky than stratus clouds, generally above 18,000 feet (5,486 m). Though rain won’t form from these clouds, which give the sky a flat, white sheen, they do tell meteorologists that rain is a distinct possibility within the coming full day. [4]

Stratocumulus

These are low-hanging clouds that also rarely mean immediate rain, even though they can appear in white, gray or nearly black. Forming below 6,000 feet, they appear in rows across the sky, often as uniform as freshly hoed crops. [5]